1. Field of the Invention
This invention relates to improvements in a surface-mounting plastic-molded type semiconductor device having a lead frame, insulating circuit board, and bonding wires.
2. Description of the Related Art
FIG. 1 shows a conventional plastic-molded type semiconductor device. FIG. 2 is a sectional view taken along line X--X' of FIG. 1. In FIGS. 1 and 2, numerals 1-1 and 1-2 indicate semiconductor elements, 2 an island, 3 an insulating circuit board, 4 wires, 5 bonding wires, 6 a molding resin, 7 inner leads, 8 a semiconductor element adhesive, and 9 an insulating circuit board adhesive.
In the semiconductor device, the semiconductor elements 1-1 and 1-2 have the same substrate potential. The semiconductor device has the insulating circuit board 3 on the surface of which a plurality of wires 4 are formed. The semiconductor elements 1-1 and 1-2 are connected to each other by the wires 4 and bonding wires 5. This arrangement prevents shorts even when wiring is complicated.
FIG. 3 shows another conventional plastic-molded type semiconductor device. FIG. 4 is a sectional view taken along line X--X' of FIG. 3. In FIGS. 3 and 4, the same parts as those in FIGS. 1 and 2 are indicated by the same reference characters.
In this semiconductor device, the semiconductor elements 1-1 and 1-2 differ in substrate potential. The semiconductor device has a plurality of islands 2-1 and 2-2, on which the semiconductor elements 1-1 and 1-2 are mounted, respectively.
In the semiconductor device of FIGS. 1 and 2, however, the insulating circuit board 3 is mounted on the island 2. Since in the semiconductor device of FIGS. 3 and 4, the insulating circuit board 3 is mounted on either of the islands, for example, on the island 2-2, the island area must be large enough to accommodate both the semiconductor device 1-2 and the insulating circuit board 3. It is known that an increase in the island area causes the shortcomings described below in mounting the semiconductor device.
Plastic-molded type semiconductor devices are generally mounted on a printed board by a technique such as solder dipping, infrared reflow, vapor phase, or hot air. During the mounting process, the semiconductor devices are heated rapidly, so that thermal stress occurs. This thermal stress vaporizes water accumulated between the island 2 and the molding resin 6, which causes cracks 10 in the resin as shown in FIG. 5. The cracks often reach the surface of the resin 6, which impairs the humidity resistance of the semiconductor device, making the device defective.
The occurrence of resin cracks 10 has relation to the island area 2 and resin thickness as shown in FIG. 6. That is, the greater the island area 2, or the thinner the resin 6 thickness, the more easily resin cracks 10 take place.
However, surface-mounting type semiconductor devices are thinner than DIPs (Dual-in-Line Plug Packages) or the like, so that resin cracks 10 occur more easily. Thus, to prevent resin cracks 10 from taking place, some measures must be taken. An increase in the resin thickness of surface-mounting type semiconductor devices goes against a tendency to make products lighter, thinner, shorter, and smaller, which fails to meet the users' needs and produce thinner packages. Consequently, it is not desirable to thicken the resin 6 of the semiconductor devices.